As described in 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration Operating Modes, ultrafiltration processes require regular backflushing to maintain their performance. This is the case in both Pressure Driven In-to-out (PDI) and Pressure Driven Out-to-in (PDO) formats.
The hydrodynamics in backwashing PDI-Membranes are much more effective than in PDO formats caused by the fact that concentrate is flushed out through a well defined (circular) flow channel in PDI, whereas "dead-zones" with very low flow velocity exist outside of the hollow fiber membranes at the bottom of PDO modules at backwash.
To provide a sufficient cleaning effect to the separation layer on the outside of a PDO membrane, air is injected to improve the removal of the inhibiting debris layer from the membrane (air scouring). This and also the fact that the feed flow is applied from the shell side of the housing require a membrane material that provides sufficient elasticity to overcome the mechanical impacts of the typical operation. PDO formats therefore largely use PVDF with its high elasticity as a membrane material but with some significant disadvantages such as a wider pore size distribution and lower permeability.
A PDI format as 3M's UltraPES does not require air scouring and by design the hydrodynamic conditions create a superior backwashing performance. It is therefore possible to use Polyethersulfone (PES) as base membrane material which provides excellent mechanical strength at high permeability and a narrow pore size distribution accompanied by the ability to generate a distinctive and functional layer structure.
As a consequence PDO formats made of PVDF serve mostly as microfiltration membranes with a lower retention and a higher transmembrane pressure to generate a sufficient filtrate flow. This generally leads to higher operating costs compared with PDI-formats.
The PDI format of 3M’s 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration was chosen to provide durable, high performance by superior hydrodynamics and low operating costs. The latter is due to low transmembrane pressure enabled by PES as the membrane material.
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